1. Field of the Invention
The present invention relates to a gas concentration meter system for measuring the concentration of a gas or a dissolved gas in liquid phase, and particularly relates to a galvanic cell type gas sensor system capable of detecting a plurality of different gasses.
2. Description of the Related Art
Galvanic cell type, or polaro type, gas sensors which operate at ordinary temperatures so as to be easy to handle are generally used for measuring the amount of a specific gas in air or water.
The galvanic cell type sensor is basically constituted by a working electrode composed of a catalyst electrode for provoking an electrochemical oxidation/reduction reaction on an objective gas, a diaphragm for limiting the supply of the gas, a counter electrode formed from reducing/oxidizing active materials, an electrolytic solution, and a container for containing the aforementioned members. Gas concentration is determined from the value of a current by taking advantage of the phenomenon that the current between a working electrode and a counter electrode is proportional to the gas concentration when electrical discharge occurs between the working electrode and the counter electrode through a resistor selected to provoke an electrochemical oxidation/reduction reaction in the diffusion-controlling rate region of the objective gas.
The gas equilibrium potential varies according to the type of gas. In the galvanic cell type gas sensor in which the potential of the working electrode greatly depends on the potential of the counter electrode, selection of the counter electrode is very important. This is so because if the counter electrode is not properly selected, it is impossible to obtain the aforementioned diffusion-controlling rate region. Also, in such a case, oxygen or hydrogen may be produced from the working electrode so as to allow current flow regardless of the concentration of the objective gas. Therefore, in the case where a plurality of gasses are to be detected, a plurality of sensors having various counter electrodes and various electrolytic solutions must be respectively prepared, each corresponding a type of gas to be detected.
A polaro type sensor is the same in basic structure as the galvanic cell type gas sensor described above, but different from the latter in that the operating potential of the working electrode is set by using an external variable voltage supply. Therefore, the potential of the working electrode can be selected freely so as to establish a diffusion-controlling rate region and avoid the production of oxygen or hydrogen. Accordingly, detection of a plurality of gasses can be performed by using one sensor.
The galvanic cell type gas sensor which theoretically utilizes a battery reaction provides an output by itself, so that it is unnecessary to use an external electric power source for driving the sensor. In order to detect a plurality of gasses as described above, however, a plurality of sensors having different counter electrodes are required. On the other hand, the polaro type gas sensor requires a constant potential application device such as a potentiostat which is expensive and difficult to handle as well as an electric source for driving the device. However, with a polaro type sensor a plurality of gasses can be detected by one sensor by changing the potential of the working electrode.